I. Technical Field
The present invention relates to an information recording medium for optically or electrically recording and/or reproducing information and a method for manufacturing the same.
II. Description of the Related Art
A conventional information recording medium has been commercialized as having a 4.7 GB/DVD-RAM, a large storage capacity, rewritable, phase-change information recording medium, usable for a data file and an image file (e.g., refer to Japanese Patent Application Laid-Open (JP-A) No. 2001-322357). The configuration of the information recording medium (DVD-RAM) is shown in FIG. 5. The information recording medium shown in FIG. 5 has a seven layer structure including a first dielectric layer 102, a first interface layer 103, a recording layer 4, a second interface layer 105, a second dielectric layer 106, a light absorption adjusting layer 7, and a reflective layer 8 on one surface of a substrate 1 in this order. With respect to this information recording medium, the first dielectric layer 102 is positioned closer to the laser incident side than the second dielectric layer 106. The first interface layer 103 has the same positioning relation with the second interface layer 105. As described above, in this specification, when an information recording medium includes two or more layers having the same function, ordinal numbers; first, second, third, and so on; are assigned in this order from the laser incident side.
The first dielectric layer 102 and the second dielectric layer 106 have the functions of adjusting the optical distance so as to increase optical absorption efficiency of the recording layer 4 and increasing the difference in reflectance between the crystal phase of the recording layer 4 and the amorphous phase of the recording layer 4 so as to magnify the signal amplitude. A mixed material of ZnS and SiO2 (in this specification, referred to as ZnS—SiO2 in some cases) which has been conventionally used for a material of a dielectric layer is an amorphous material and has a low thermal conductivity, transparency, and a high refractive index. Further, ZnS—SiO2 has a high film deposition speed at the time of film deposition and is also excellent in mechanical properties and resistance to humidity. In this manner, ZnS—SiO2 is an excellent material for a dielectric layer of an information recording medium.
If the thermal conductivity of the first dielectric layer 102 and the second dielectric layer 106 is low, the heat generated at the time of laser incidence upon the recording layer 4 is unlikely to be diffused in the in-plane direction of the dielectric layer 102 or 106 and smoothly diffused in the thickness direction from the recording layer 4 to the reflective layer 8. Particularly, when the thermal conductivity of the second dielectric layer 106 is low, the recording layer 4 and the reflective layer 8 are more thermally insulated by the second dielectric layer 106. The larger the extent of thermal insulation between the recording layer 4 and the reflective layer 8 is, the shorter the time for cooling the recording layer 4, and amorphous marks (recording marks) tend to be formed more easily. When the recording marks are difficult to form, it is required to carry out recording with high peak power. In contrast, when the recording marks are easy to form, it is possible to carry out recording with low peak power. As described above, when the thermal conductivity of the dielectric layers 102 and 106 is low, it is possible to carry out recording with low peak power and accordingly the recording sensitivity of the information recording medium can be increased.
When the thermal conductivity of the dielectric layers 102 and 106 is high, high peak power recording is required and accordingly the recording sensitivity of the information recording medium is decreased. The dielectric layers 102 and 106 of the information recording medium exist in form of such a thin film so as to make it impossible to precisely measure thermal conductivity. Therefore, the present inventors employ recording sensitivity of an information recording medium as a relative judgment standard to know the degree of the thermal conductivity of a dielectric layer.
The recording layer 4 is formed using a material containing Ge—Sn—Sb—Te and to be crystallized at a high speed. An information recording medium containing such a material for the recording layer 4 has not only excellent initial recording performance but also an excellent recording conservation property and a rewriting conservation property. A rewritable phase-change type information recording medium performs recording, erasing, and rewriting based on reversible phase change caused between the crystal phase and the amorphous phase of the recording layer 4. If high power laser light (peak power) is applied to the recording layer 4 and the recording layer is quickly cooled, the irradiated portion becomes an amorphous phase thereby forming recording marks. If lower power laser light (bias power) is applied to increase the temperature of the recording layer 4 and the recording layer is gradually cooled, the irradiated portion becomes a crystal phase thereby erasing the recorded information. Application of laser light with modulated power between the peak power level and the bias power level to the recording layer makes it possible to erase already recorded information and simultaneously rewrite new information. The repeated rewriting performance can be expressed as the maximum number of rewriting to be repeated to the extent that the jitter value is within a practically usable range. It can be said that the repeated rewriting performance is better as the repeat count is higher. Particularly, an information recording medium for a data file is desired to have excellent repeated rewriting performance.
The first interface layer 103 and the second interface layer 105 have a function of preventing mass transfer generated between the first dielectric layer 102 and the recording layer 4 and between the second dielectric layer 106 and the recording layer 4 (e.g., refer to JP-A No. 10-275360 and International Publication Pamphlet No. 97/34298). That is, a sulfur atom (hereafter referred to as an S atom) of ZnS—SiO2 contained in the first and the second dielectric layers 102 and 106 is prevented from diffusing in the recording layer 4 during repeated rewriting by laser irradiation to the recording layer 4. It has been known that if a large quantity of S atoms is diffused to the recording layer 4, the reflectance of the recording layer 4 is decreased and the repeated rewiring performance is deteriorated (e.g., refer to N. Yamada, et al. Japanese Journal of Applied Physics, vol. 37, 1998, pp. 2104-2110).
The light absorption adjusting layer 107 has functions of adjusting the ratio Ac/Aa of the optical absorbance Ac when the recording layer 4 is in a crystalline state and the optical absorbance Aa when the recording layer 4 is in an amorphous state and suppressing the strain of a mark form at the time of rewriting. The reflective layer 8 has an optical function of increasing the amount of light to be absorbed in the recording layer 4 and a thermal function of quickly diffusing heat generated in the recording layer 4 and quickly cooling the recording layer 4 to make the recording layer 4 easy to be amorphous. The reflective layer 8 also has a function of protecting the multilayer film from the environments in which the film is used.
As described above, the information recording medium shown in FIG. 5 has a configuration formed by laminating seven layers respectively having the above-mentioned functions, so that the information recording medium retains excellent repeated rewriting performance and high reliability even having a storage capacity as large as 4.7 GB.
As described above, an interface layer for reducing mass transfer is inevitably needed between a recording layer and a dielectric layer to assure the repeated rewriting performance of an information recording medium.
However, in consideration of the cost of the medium, it is desirable for the medium to have a less number of constituent layers even by a single. It is because that decrease of the number of layers can save the material cost, make a production apparatus compact and simple, and increase the productivity by shortening the production time and consequently cutting cost of the recording medium.
Further, the interface layers are very thin layers with a thickness of 2 nm to 5 nm and weak in the structure. Therefore, it is likely that the film is broken during the time of repeated recording and as a result, atom diffusion is caused easily. Consequently, elimination of the interface layers is desirable in terms of the stability of the information recording medium.